Apparently improved energy barrier of Dy(iii) SMMs by adopting the bridging bipyrimidine ligand

Eight complexes Ln(tfa)(dmpy) (Ln = Gd 1, Dy 2, tfa = 1,1,1-trifluoro-2,4-pentanedione dmpy = 4,4′-dimethyl-2,2′-bipyridyl), Ln(tfpa)(dmpy) (Ln = Gd 3, Dy 4, tfpa = 4,4,4-trifluoro-1-phenyl-1,3-butanedione), [Ln(tfa)](bpyd) (Ln = Gd 5, Dy 6, bpyd = 2,2′-bipyrimidine) and [Ln(tfpa)](bpyd) (Ln = Gd 7, Dy 8) were designed to investigate the influence of magnetic coupling transfer and changes in molecular geometry on the performance of single-molecule magnets (SMMs). The single-crystal X-ray diffraction results show that Ln(tfa)(dmpy) and Ln(tfpa)(dmpy) are mononuclear, while [Ln(tfa)](bpyd) and [Ln(tfpa)](bpyd) are binuclear complexes with bipyrimidine acting as bridging ligands. The Ln ions in these complexes are octahedral structures. Direct current (dc) susceptibility data indicate that bipyrimidine can effectively transfer the magnetic exchange of two Ln ions. The results of alternating current (AC) magnetization characterization reveal that the magnetic exchange of adjacent Dy ions can obviously suppress the quantum tunneling effect. Therefore, complexes 2 and 4 display weak magnetic relaxation with the absence of peaks. Comparatively, complexes 6 and 8 exhibit distinct slow magnetic relaxation behaviors under the same conditions, with U values of 319.38 and 173.18 K, respectively. Theoretical calculations suggest that the weak Dy-Dy dipole-dipole interaction in complex 6 can effectively hinder the magnetic quantum tunneling in the lowest four Kramers doublets (KDs) of Dy fragments. Consequently, the U value of complex 6 is notably higher than that of complex 8. These findings could provide insights for achieving higher energy barriers in binuclear Dy compounds.